Optimize model execution to stop immediately when required

[HMNS19]

This PR updates the do_step function to exit early when running.value is False. Previously, it would continue executing n steps even after running was set to stop. Now, it stops immediately when running.value becomes False.

This applies to models running in both ModelController and SimulatorController.

[github-actions]

Performance benchmarks:

Model	Size	Init time [95% CI]	Run time [95% CI]
BoltzmannWealth	small	🔵 +0.6% [+0.1%, +1.1%]	🔵 -0.1% [-0.3%, +0.2%]
BoltzmannWealth	large	🔵 +0.2% [-0.4%, +0.8%]	🔵 -1.6% [-2.4%, -0.8%]
Schelling	small	🔵 -0.4% [-0.6%, -0.2%]	🔵 -0.5% [-0.7%, -0.4%]
Schelling	large	🔵 -0.6% [-0.8%, -0.3%]	🔵 -0.5% [-1.9%, +0.7%]
WolfSheep	small	🔵 -1.2% [-1.4%, -1.0%]	🔵 -1.0% [-1.1%, -0.8%]
WolfSheep	large	🔵 -1.0% [-1.5%, -0.5%]	🔵 -0.8% [-1.1%, -0.5%]
BoidFlockers	small	🔵 +1.2% [+0.3%, +2.1%]	🔵 +3.2% [+3.0%, +3.4%]
BoidFlockers	large	🔵 +0.9% [+0.5%, +1.2%]	🔵 +2.4% [+2.1%, +2.6%]

[HMNS19]

I noticed that the checks didn’t pass in my PR due to an AttributeError in the test. The issue seems to be that model.running isn’t set in the mock object for simulator models. Since I added a feature to stop the model when running is False, the test fails because it tries to access model.running, which doesn’t exist in the mock.

Would it make sense to update the test setup to include model.running = True, or should we handle this differently?